Machines such as articulated haul trucks and off-highway mining trucks include an engine that provides power to wheels of the trucks via a planetary-type transmission. A planetary-type transmission is generally made up of at least three different elements, including a sun gear, a planet carrier having at least one set of planet gears, and a ring gear. The planet gears of the planet carrier mesh with the sun gear and the ring gear. One of the sun gear, planet carrier and ring gear is driven as an input to the transmission, while another of the sun gear, planet carrier, and ring gear rotates as an output of the transmission. The sun gear, planet carrier, planet gears, and ring gear can all rotate simultaneously to transmit power from the input to the output at a first ratio of speed-to-torque and in a forward direction or, alternatively, one of the sun gear, planet carrier, and ring gear can be selectively held stationary or locked to rotate with another gear and thereby transmit power from the input to the output at a second ratio of speed-to-torque and/or in a reverse direction. The change in rotational direction and/or speed-to-torque ratio of the transmission depends upon the number of teeth in the sun and ring gears, the gear(s) that is selected as the input, the gear(s) that is selected as the output, and which gear, if any, is held stationary or rotationally locked with another gear. A hydraulic clutch (also commonly referred to as a brake) is used to hold particular gears stationary and/or to lock the rotation of particular gears together.
The amount of resistive torque required to hold a gear stationary or to lock the rotation of one gear together with another gear can vary. For example, when operating in a low reverse gear, significantly more torque may be required from a particular clutch than required from the same clutch during operation in a forward high gear. When the clutch is capable of providing only a single level of torque (i.e., the highest level required during any operation), shifting of the transmission between reverse and forward gears when only a small amount of resistive torque is required can be too rapid, resulting in rough operation that is uncomfortable for an operator of the machine and possibly damaging to the machine.
One attempt to improve shift quality is described in U.S. Pat. No. 7,140,481 (the '481 patent) by Hagenow that issued on Nov. 28, 2010. Specifically, the '481 patent discloses a clutch piston amplifier assembly provided for applying different levels of pressure to a clutch pack to engage a first member and a second member of an automatic transmission. The clutch amplifier assembly includes a piston chamber, and a stage separator plate provided in the piston chamber to divide the piston chamber into a first chamber portion and a second chamber portion. The clutch amplifier assembly also includes an apply piston provided in the first chamber portion and having an axially extending arm which engages the clutch pack for applying axial pressure thereto, and an amplifier piston provided in the second chamber portion. Amplifier drive pins are provided between and engaged with both the apply piston and the amplifier piston, and a return spring is located between the apply piston and a wall of the piston chamber. During operation, pressurized fluid is directed into the first chamber portion to move the apply piston against the spring and generate a first pressure on the clutch pack (i.e., a first resistive torque on the first and second members). Pressurized fluid can also or alternatively be directed into the second chamber portion to move the amplifier piston against the drive pin, which pushes on the apply piston to increase the pressure or provide a different level of pressure on the clutch pack. In this manner, three levels of pressure (apply piston pressure, amplifier piston pressure, or apply piston pressure+amplifier piston pressure) are available, thereby allowing for tunability of the pressure and a resulting smoother shift operation.
Although the assembly of the '481 patent may improve shift quality, it may be complicated and difficult to control. Specifically, different valving for each of the first and second chamber portions may be required to appropriately control the flows of pressurized fluid. In addition, regulation of the different valving may require precise timing and complex control. Further, use of the amplifier drive pins may increase a cost of the system, provide a potential leak path for pressurized fluid, and reduce a durability of the system.
The disclosed transmission assembly is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.